I wanted a nicer way without installing manually and using only Homebrew (which also does not link openssl by default). Also using pip was a requirement. This seems to work with newest m2crypto 0.22.5. I also tested it once with m2crypto 0.22.3 and seems also to work. The OpenSSL version here is 1.0.2d:
brew install openssl
brew install swig
Finally install m2crypto on macOS in your Bash. It is a long command but it changes SWIG and clang environment variables only during pip install so that m2crypto will get all OpenSSL requirements:
env LDFLAGS="-L$(brew --prefix openssl)/lib" \
CFLAGS="-I$(brew --prefix openssl)/include" \
SWIG_FEATURES="-cpperraswarn -includeall -I$(brew --prefix openssl)/include" \
pip install m2crypto
btw. the last command also works if you use e.g. a requirements.txt.
Update:
Additional also the command for fish shell users…
env LDFLAGS="-L"(brew --prefix openssl)"/lib" \
CFLAGS="-I"(brew --prefix openssl)"/include" \
SWIG_FEATURES="-cpperraswarn -includeall -I"(brew --prefix openssl)"/include" \
pip install m2crypto